Cleaning aluminum heat exchange surfaces

ABSTRACT

IN A FURFURAL REFINING PROCESS THE AVERAGE HEAT EXCHANGE CAPACITY OF AN ALUMINUM HEAT EXCHANGE SURFACE EMPLOYED TO TRANSFER HEAT BETWEEN PROCESS STREAMS MAY BE IMPROVED BY CLEANING RELATIVELY SOFT ACCUMULATED DEPOSITS FROM SUCH SURFACE IN A MANNER SUCH THAT THIN. RELATIVELY HARD DEPOSITS ADJACENT THE HEAT EXCHANGE SURFACE IS SUBSTANTIALLY UNDISTURBED.   D R A W I N G

Feb. 6, 1973 C.A. STEVENS 3,714,979

CLEANING ALUMINUM mfwr mxcnANm': suRvAcl-:s

Filed Aug. 25, 971

United States Patent O 3,714,979 CLEANING ALUMINUM HEAT EXCHANGE SURFACES Clifford A. Stevens, Port Neches, Tex., assignor to Texaco Inc., New York, N.Y. Filed Aug. 25, 1971, Ser. No. 174,590 Int. Cl. F28g 9/00 U.S. Cl. 165-1 8 Claims ABSTRACT OF THE DISCLOSURE In a furfural refining process the average heat exchange capacity of an aluminum heat exchange surface employed to transfer heat between process streams may be improved by cleaning relatively soft accumulated deposits from such surface in a manner such that thin, relatively hard deposits adjacent the heat exchange surface is substantially undisturbed.

BACKGROUND OF THE INVENTION This invention relates to furfural refining of petroleum fractions wherein undesirable materials such as acidic, sulfur, organo-metallic, nitrogen, and aromatic compounds are extracted from petroleum oils. More particularly, this invention concerns a method for treating heat exchangers to improve heat transfer between process streams in a commercial furfural refining unit.

Furfural refining of petroleum fractions to produce petroleum oils with improved properties is well known in the prior art. For instance, gas-oil charge stock may be furfural refined to produce diesel fuel with an improved cetane number and lubricating oil stock may be furfural refined to improve temperature-viscosity characteristics of the refined lubricating oil stock, see Hydrocarbon Processing, 1970 Refining Handbook, September 1970, pp. 23S-239.

A furfural refining process may comprise contacting a petroleum charge stock, such as a lubricating oil stock, countercurrently with a furfural stream in a treating tower wherein undesirable components are extracted from the petroleum charge stock into the furfural stream. Treated petroleum charge stock is recovered as a product overhead from the treating tower. From the bottom of the tower an extract stream comprising furfural and extracted components of the petroleum charge stock is recovered. Furfural may be recovered from the extract stream by heating and flashing the extract stream in an extract pressure fiash tower. A vapor stream comprising furfural is recovered from the extract pressure flash tower and charged to a rt'urfuratl separator. A liquid product comprising furfural and extracted components is recovered from the extract pressure liash tower and may be charged to an extract atmospheric flash tower. From the extract atmospheric flash tower a vapor stream comprising furfural is recovered, and charged to the furfural separator. A liquid stream comprising extracted components is recovered from the extract atmospheric flash tower. A portion of the extracted component stream recovered from the extract atmospheric flash tower may be returned to the bottom of the treating tower as extract reflux. All the extracted component stream not returned to the treating tower may be removed from the fnrfural process as an extract product.

Furfural is recovered from the extract pressure flash tower vapor stream and from the extract atmospheric flash vapor stream in the furfural separator as a bottom liquid product. An overhead vapor stream comprising furfural, water, and low boiling petroleum oils is recovered from said separator. The separator overhead vapor stream is condensed and condensate is passed into a settler wherein furfural is separated from the other components by gravity settling. Fnrfural from the settler is returned as overhead liquid refiux to the furfural stripper. Furfural from the furfural separator is recycled to the treating tower wherein it is employed to extract undesirable components from additional petroleum charge stock.

In commercial furfural refining processes, it is common practice to conserve energy by exchanging heat between the various process streams. For example, heat may be exchanged from the extract atmospheric flash tower vapor stream to the treating tower extract stream. Also heat may be exchanged from the extract pressure flash tower vapor to the treating tower extract stream. Heat may also be exchanged between other process streams. For instance, heat may be exchanged between furfural recovered from the furfural separator and the petroleum charge entering the treating tower. Such heat transfer between other process streams may also be employed where the temperature differential is adequate to allow economical heat transfer. All such heat transfer between process streams is generally accomplished by indirect heat transfer means, such as for example, shell and tube heat exchangers.

Heat exchangers employing aluminum heat transfer surfaces, such as aluminum tubes in shell and tube heat exchangers, have been found to be particularly advantageous in commercial furfural refining processes, Since many process streams in the furfural refining process are not severely corrosive to aluminum, the excellent heat transfer coefficient and light weight of aluminum may be used to advantage in heat exchangers employed in a furfural refining process.

In a furfural refining process wherein process streams are employed as heat exchange media in heat exchangers, it has been found that solid deposits primarily comprising organic materials accumulate upon the heat exchange surfaces. As the accumulation of these deposits increases, the heat exchange capacity of the heat exchangers decreases. Therefore, periodically such heat exchangers are removed from the furfural refining process and the accumulated deposits are removed from the heat exchange surfaces. Removal of these accumulated deposits restores the heat transfer capacity of the heat exchange surface. Heat exchangers with the accumulated deposits removed therefrom are subsequently returned to service in the furfural refining process.

The accumulated deposits on the heat exchange surfaces comprise a very thin, tough film immediately adjacent the heat exchange surface, and overlying said film, a thicker layer of soft accumulated deposits primarily composed of organic materials. It is known that the thin film adjacent to the heat exchange surface contributes substantially to the decrease in heat exchange capacity of said heat exchange surface. On the basis of thickness, the thin film contributes an inordinate amount to the decrease in heat exchange capacity as compared to the thicker soft deposit which overlies the thin film. Consequently, the common practice in cleaning heat exchangers removed from a furfural refining process has been to insure that the thin film is substantially completely removed from the heat exchange surface before returning the cleaned heat exchangers to service in the process. Such means as hydroblasting with a water and sand mixture and other severe cleaning methods are commonly employed to insure that substantially all the thin film deposits are removed from the heat exchange surfaces.

SUMMARY OF THE INVENTION Now in accordance with the present invention I have discovered that the heat transfer characteristics of a heat Patented Feb. 6, 1973.,`

exchanger comprising an aluminum heat transfer surface employed in a furfural refining process may be substantially improved. More particularly, the heat exchange capacity of a heat exchanger comprising an aluminum heat transfer surface cleaned according to the method of the present invention is maintained at a high value for a longer period as compared to heat exchangers cleaned according to the methods of the prior art.

The advantages of applying the heat exchanger cleaning method of the present invention is that a heat exchanger, so cleaned, may be employed in a furfural refining process for a period substantially longer than either new heat exchangers or heat exchangers cleaned according to the methods of the prior art. More heat energy may be conserved by employing the method of the present invention and labor costs for removing, cleaning and reinstalling heat exchangers may be substantially reduced.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a graphical example demonstrating the comparative improvement of heat exchange capacity for a heat exchanger cleaned according to the method of the present invention over the same exchanger when new and when cleaned according to a prior art method.

DETAILED DESCRIBED OF THE INVENTION In a furfural refining process as hereinbefore described, wherein heat is exchanged between streams of said process employing a heat exchanger comprising an aluminum heat transfer surface, it has been noted that the heat transfer capacity of such heat exchanger decreases with time due to the accumulation of deposits upon such heat transfer surface. I have discovered a method for cleaning a heat exchanger removed from such a process such that When the cleaned heat exchanger is returned to service in the process the average heat exchange capacity over an extended period of time is substantially greater than the average heat exchange capacity for a similar heat exchanger cleaned according to methods known to the prior art.

According to the method of the present invention, a heat exchanger removed from a furfural refining process 'which has deposits primarily comprising organic material accumulated upon the aluminum heat exchange surface thereof is cleaned in a manner such that the thick, relatively soft, layer of accumulated deposits is substantially completely removed and such that the thin film of deposits immediately adjacent the heat exchange surface are substantially left intact. For example, such soft accumulated deposits may be removed from a heat exchange surface by washing said surface with a high pressure stream of water. Such soft deposits may be removed from the heat exchange surfaces by alternative methods such as, for exxample, washing with high pressure steam, and other methods which remove the relatively soft deposits and leave the thin film deposit substantially undisturbed.

To demonstrate the improvement of the present invention, attention is now drawn to the attached drawing. FIG. 1 of the drawing is a graphical representation of the heat transfer capacity, expressed as the heat transfer coefficient with units of B.t.u.s per hour per degree F. per square foot of heat exchange surface, plotted as a function of time for a shell and tube heat exchanger comprising aluminum heat transfer tubes. This heat exchanger was employed to transfer heat from an extract atmospheric flash tower vapor stream to the treating tower extract effiuent stream. Line 1 of the graphical representation shows the decline in heat transfer coefiicient with respect to time on stream for this exchanger during a process period beginning when the exchanger was new and had not previously been used. Line 2 shows the decline of heat transfer coeicient with respect to time on stream for this exchanger during a process period beginning when the exchanger, hydroblasted to remove substantially all accumulated deposits including the thin film deposit adjacent to heat transfer surface, was returned to service in the furfural refining process. Line 3 shows the decline of heat transfer coefficient with respect to time on stream for this exchanger during a process period beginning when the exchanger, cleaned according to the method of the present invention was washed with a high pressure water stream to remove the soft accumulated deposits from the heat exchange surface without disturbing the thin film deposit, was returned to service. From an examination of the drawing, it can be seen that the heat transfer coeflicient for the exchanger cleaned according to the method of the present invention, although somewhat lower at the beginning of service, after a service period of less than about days surpassed the heat transfer coefficient of the exchanger when cleaned by hydroblasting or when new. It can also be seen that the heat transfer coefficient of the exchanger cleaned according to the method of the present invention remained greater during a process pe riod from 80 days to 360 days, at which time the heat exchanger was removed from service in the furfural refining process. The average heat transfer coefficient for the exchanger when cleaned according to the method of the present invention during its period of service in the furfural refining process was substantially higher than the average heat transfer coefficient for the exchanger when hydroblasted or of the exchanger when it was new.

The reason cleaning according to the method of the present invention improves the average heat transfer capacity of a heat exchanger comprising an aluminum heat transfer surface is not fully understood and no theoretical explanation has been proposed. The observed results of improved heat exchange capacity obtained by leaving the thin film deposit on the aluminum heat exchange surface substantially undisturbed is unexpected. Experience in cleaning heat exchange surfaces comprising other metals, such as steel, has shown that the thin film deposits must be removed from the heat exchange surfaces in order to restore an acceptable heat exchange capacity to such surfaces.

Those skilled in the art will determine methods for cleaning heat exchangers which are within the spirit and scope of the present invention. Therefore, no limitation to the invention is to be implied except those contained in the appended claims.

I claim:

1. In a furfural refining process wherein heat is exchanged between process streams employing a heat exchanger comprising an aluminum heat transfer surface, wherein solid deposits accumulate upon said heat transfer surface, wherein such heat exchanger is periodically removed from said process to remove the accumulated deposits, and wherein the cleaned heat exchanger is subsequently returned to service in the process; the improvement in cleaning the heat transfer surface prior to returning the heat exchanger to service in the process which comprises:

cleaning the heat exchange surface in a manner such that substantially all the relatively soft deposits are removed and such that the thin film deposit immediately adjacent to the heat transfer surface remain substantially intact.

2. The method of claim 1 wherein the heat exchange surface is cleaned by washing with a stream of high presssure water.

3. A furfural refining process, which comprises:

extracting undesirable components from a petroleum fraction Iwith liquid furfural; separating a treated petroleum fraction from an extract phase comprising furfural and unwanted components; vaporizing furfural from said extract phase; exchanging heat from said furfural vapor to said unvaporized extract phase employing a heat exchanger comprising an aluminum heat transfer surface wherein said surface has been cleaned in a manner such that a thin film deposit adjacent the heat transfer surface remains substantially intact; condensing said furfural vapor; and recycling the condensed furfural through the extraction step above.

4. The method according to claim 1 wherein the heat exchanger is employed to exchange heat from an extract pressure flash tower effluent vapor to a treating tower extract eiuent stream.

5. The method according to claim 1 wherein the heat exchanger is employed to exchange heat from an extract atmospheric ash tower effluent vapor to a treating tower extract effluent stream. t 6. In a furfural refining process wherein heat is exchanged between process streams employing a heat exchanger comprising an aluminum heat transfer surface, wherein deposits comprising a thin lm immediately adjacent the heat transfer surface and a relatively soft layer overlying said thin film accumulate upon the heat transfer surface, wherein the heat exchanger is periodically removed from service for removal of accumulated deposits from said heat transfer surface, and wherein the cleaned heat exchanger is subsequently returned to service in the process, which process is characterized by a decrease in heat transfer capacity of the heat exchanger heat transfer surface relative to the period the exchanger has been in service in said process;

. the improvement in cleaning accumulated deposits from a heat transfer surface prior to returning said heat transfer surface to service in the process, which comprises:

removing substantially all the relatively soft accumulated deposits from said heat transfer surface; and leaving the thin film deposit adjacent the heat transfer surface substantially intact.

7. In a furfural refining process wherein heat is exchanged between process streams employing a heat ex changer comprising an aluminum heat transfer surface, wherein deposits comprising a thin film immediately adjacent the heat transfer surface and a relatively soft layer overlying the thin film accumulate upon the heat transfer surface, wherein the heat exchanger is periodically removed from service for removal of accumulated deposits from the heat transfer surface, and wherein the cleaned heat exchanger is subsequently returned to service in the process. which process is characterized by a decrease in heat transfer capacity of the heat exchanger heat transfer surface relative to the period of time the exchanger has been in service in said process;

the improvement for increasing the average heat transfer capacity of a cleaned heat transfer surface, which comprises:

removing substantially all the relatively soft accumulated deposits from said heat transfer surface; and leaving the thin film deposit adjacent the heat transfer surface substantially intact.

8. `In a furfural refining process wherein heat is exchanged between process streams employing a heat exchanger comprising an aluminum heat transfer surface, wherein deposits comprising a thin film immediately adjacent the heat transfer surface and a relatively soft layer overlying the thin film accumulate upon the heat transfer surface, wherein the heat exchanger is periodically removed frorn service for removal of accumulated deposits from the heat transfer surface, and wherein the cleaned heat exchanger is subsequently returned to service in the process, which process is characterized by a decrease in heat transfer capacity of the heat exchanger heat transfer surface relative to the period of time the exchanger has been in said process;

the improvement for decreasing the rate at which the heat transfer capacity of a cleaned heat transfer surface returned to service n the process declines with time, which comprises:

removing substantially all the relatively soft de posits from a heat exchanger surface in a manner wherein the thin film deposits adjacent the heat exchange surface are substantially undisturbed prior to returning said heat exchange surface to service in the process.

References Cited UNITED STATES PATENTS MANUEL A. ANTONAKAS, Primary Examiner U.S. Cl. XR. 

